Some cleanup of st_get_pos functions

Marlin/planner.cpp

 } // plan_buffer_line()
 
 #if ENABLED(AUTO_BED_LEVELING_FEATURE) && DISABLED(DELTA)
+
+  /**
+   * Get the XYZ position of the steppers as a vector_3.
+   *
+   * On CORE machines XYZ is derived from ABC.
+   */
   vector_3 plan_get_position() {
     vector_3 position = vector_3(st_get_axis_position_mm(X_AXIS), st_get_axis_position_mm(Y_AXIS), st_get_axis_position_mm(Z_AXIS));
 
 
     return position;
   }
+
 #endif // AUTO_BED_LEVELING_FEATURE && !DELTA
 
+/**
+ * Directly set the planner XYZ position (hence the stepper positions).
+ *
+ * On CORE machines stepper ABC will be translated from the given XYZ.
+ */
 #if ENABLED(AUTO_BED_LEVELING_FEATURE) || ENABLED(MESH_BED_LEVELING)
   void plan_set_position(float x, float y, float z, const float& e)
 #else

Marlin/stepper.cpp

   CRITICAL_SECTION_END;
 }
 
-long st_get_position(uint8_t axis) {
+/**
+ * Get a stepper's position in steps.
+ */
+long st_get_position(AxisEnum axis) {
   CRITICAL_SECTION_START;
   long count_pos = count_position[axis];
   CRITICAL_SECTION_END;
   return count_pos;
 }
 
+/**
+ * Get an axis position according to stepper position(s)
+ * For CORE machines apply translation from ABC to XYZ.
+ */
 float st_get_axis_position_mm(AxisEnum axis) {
-  float axis_pos;
+  float axis_steps;
   #if ENABLED(COREXY) | ENABLED(COREXZ)
     if (axis == X_AXIS || axis == CORE_AXIS_2) {
       CRITICAL_SECTION_START;
       CRITICAL_SECTION_END;
       // ((a1+a2)+(a1-a2))/2 -> (a1+a2+a1-a2)/2 -> (a1+a1)/2 -> a1
       // ((a1+a2)-(a1-a2))/2 -> (a1+a2-a1+a2)/2 -> (a2+a2)/2 -> a2
-      axis_pos = (pos1 + ((axis == X_AXIS) ? pos2 : -pos2)) / 2.0f;
+      axis_steps = (pos1 + ((axis == X_AXIS) ? pos2 : -pos2)) / 2.0f;
     }
     else
-      axis_pos = st_get_position(axis);
+      axis_steps = st_get_position(axis);
   #else
-    axis_pos = st_get_position(axis);
+    axis_steps = st_get_position(axis);
   #endif
-  return axis_pos / axis_steps_per_unit[axis];
+  return axis_steps / axis_steps_per_unit[axis];
 }
 
 void finishAndDisableSteppers() {

Marlin/stepper.h

 void st_set_e_position(const long& e);
 
 // Get current position in steps
-long st_get_position(uint8_t axis);
+long st_get_position(AxisEnum axis);
 
 // Get current axis position in mm
 float st_get_axis_position_mm(AxisEnum axis);